You're not wrong about their anti-slavery history/Reconstruction era, but OP isn't either. OP is apparently referring to the pro big business bent of the Republican party, which long ago (including then) led the party to favor expansion of the government.
Despite having a small government platform today, modern Republicans are known as the party of the wealthy now, and recent studies suggest the biggest slaveowners remained wealthy after the war. My educated guess is that many of these plantation dynasty people since switched parties (to Republican), and we know Democrats do well among descendants of slaves. You're both right; OP makes a better point imo. 😥
https://www.washingtonpost.com/us-policy/2019/04/04/how-souths-slave-owning-dynasties-regained-their-wealth-after-civil-war/